This invention relates to a refractive index fluid for optics and more particularly to such a fluid which can be employed as a coupling adhesive in the optical field.
The image of a specimen viewed through a microscope is very much effected by the refractive index and the optical dispersion (the change in refractive index with the wavelength of light) of the medium in which the specimen is mounted. For example, a colorless specimen will appear invisible unless it is a medium of a different refractive index from the specimen, and it will appear opaque and featureless if that difference in refractive index is too great. Also the identity of a specimen such as a mineral or glass can often be made by viewing it in a medium of a known but different optical dispersion and observing the resultant characteristic colors around the boundaries of the microscopic image.
Mounting media in microscopy are generally applied with the specimen to a glass slide and a thin glass cover slip is then applied so that the specimen is then sandwiched with the medium between the cover glass and the slide. For a "permanent slide" the medium is also an adhesive.
For example of such techniques reference is made to U.S. Pat. No. 3,445,152 entitled MICROSCOPE SLIDES AND METHOD OF MANUFACTURING issued on May 20, 1969 to S. B. Carter. This patent describes a particular type of slide using resins to affix the specimen.
Other patents as U.S. Pat. No. 3,736,042 entitled MICROSCOPE SLIDE ASSEMBLY issued on May 29, 1973 and shows other constructions and materials employed with such slides.
Other patents as U.S. Pat. No. 3,891,327 entitled MOUNTED SLIDES AND METHOD OF SECURING A COVER GLASS TO A GLASS SLIDE AND HAVING A SPECIMEN THEREON issued on June 14, 1975 to M. J. Welch. This patent shows a cover glass bonded to the glass slide in a fixed position by a photosensitive composition.
Most mounting media used by microscopists are essentially resins dissolved in a solvent. The main problems with solvent type media is, the very long time required to dry out the solvent to form a "permanent slide", change of refractive index as the medium dries, shrinkage and bubble formation upon drying, and cracking or crystallizing with drying or aging. The melt or fluid type media avoid these problems because the prepared slide does not change from the time it is made because there is no drying. The melt type medium is commonly heated until it is fluid on a hotplate along with the slide and specimen; the medium is applied over the specimen on the slide, the cover glass is affixed, and the slide is allowed to cool and form a "permanent slide" with the melt functioning as a resinous adhesive. Unlike the solvent type media, the slide can be reheated and the specimen removed if necessary. The fluid type medium can be used to make "semi-permanent slides". The specimen is sandwiched with a drop or two of the fluid medium between the cover slip and slide. This should not be confused with Cargille Refractive Index liquids which have a much lower viscosity. The fluid type medium has a very high viscosity for a liquid and the slide made using it is permanent with the cover slip secured well in place as long as the slides are stored in a flat rather than an upright position. The advantage of this type of medium is that while it secures the cover slip to protect the specimen during handling, the cover slip can be moved with a finger to rotate particles (such as diatoms and crystals) to change orientation for viewing. This rotation of the specimen by moving the coverslip with a finger is commonly done using Cargille Refractive Index liquids but such slides must be handled carefully because of the low viscosity of these liquids and are usually not considered "permanent or semi-permanent slides".
At one time the most common melt type medium was a material with the tradename Aroclor 5442. Aroclor 5442 was used for years and many reference substances used by microscopists were mounted in it, until the United States Environmental Protection Agency banned the substance because it might contain polychlorinated biphenyls (PCB's). The problem in finding a replacement for this material is that not only that high refractive index resins are scarce (Aroclor 5442 has a refractive index of 1.66) but the replacement must match the dispersion of it so that the vast sets of reference materials that microscopists have already mounted in Aroclor 5442 will be usable for comparison with materials mounted in the replacement material. Since the ban on Aroclor 5442 the only suitable high index mounting media have been Hyrax (refractive index 1.63) and Naphrax (refractive index 1.7) both of which are solvent based and hard to find.
U.S. Pat. No. 3,929,667 issued on Dec. 30, 1975 to A. N. Bautis and entitled IMMERSION OIL FORMULATIONS FOR USE IN MICROSCOPY AND SIMILAR FIELDS and assigned to the assignee herein. This patent describes a non-toxic immersion oil which was employed to replace oils containing PCBs.
In the field of optics there has long been a need for high refractive index adhesive material to join optical components. A relatively recent need is for a medium to reversibly join optical fibers that will have the necessary refractive index and optical properties to reduce a signal loss.